In a discharging battery, the conversion of chemical potential energy into useful electrical energy occurs by electron flow from the negative electrode, or fuel electrode acting as the anode, through an external circuit to the positive electrode, acting as the cathode. Simultaneously, ions flow through the ionically conductive medium. In a rechargeable battery, which is also known as a secondary battery, the chemical reactions can be reversed by connecting the cell to an external power supply with electron flow from the cathode to the anode. To a large degree, both the amount of energy contained in a secondary battery and the number of times it can be recharged determine its economic value.
Metal fuel negative electrodes may be paired with any number of positive electrodes, one example being an air electrode. Metal-air cells are well-known, and comprise a metal fuel electrode and an air electrode. During discharge, the metal fuel is oxidized at the metal fuel electrode and oxygen is reduced at the air electrode. In metal-air cells of the rechargeable or secondary type, the metal fuel may be reduced on the fuel electrode, and oxygen may be evolved by oxidation at the air electrode or a separate charging electrode. Metal-air electrochemical cells are able to combine the ultra-high anode capacity of batteries with the air-breathing cathode of fuel cells in order to achieve substantial energy densities that are relevant to modern energy demands.
The passivation layer can act as a protective film to minimize unfavorable corrosion during idle battery states, but by the same token, it can act as a barrier to desired corrosion (i.e. during the discharge of the battery). During discharge, the metal fuel may be oxidized to form a passivating film, the composition of which is dependent on battery chemistry. For example, zinc oxide is formed in zinc-air batteries. In a battery, this layer can passivate the metal of the fuel electrode which impedes the essential electrochemical reactions occurring at this electrode. If a constant anodic current is applied to the fuel electrode, the overpotential needed to maintain the current may increase with time, thereby drawing an increasing amount of parasitic power.
Doped oxides have been used as conductive additives in electrochemical cells to increase conductivity at both negative and positive electrodes. For example, U.S. Pat. No. 6,524,750 filed Jun. 27, 2000 teaches various doped oxide additives, specifically high conductivity oxides of differing metals than the metal fuel anode. In a similar example, U.S. Pat. No. 6,818,347 filed on Mar. 22, 2001 describes incorporation of conductive n-type doped oxide additives, specifically niobia-doped TiO2 as an additive for Zn/MnO2 batteries wherein Zn is the metal fuel.
Suppression of dendritic growth upon by semiconductor oxide layers upon metal fuel electrodes has been observed, for example Yang et al., “Effect of La Addition to the Electrochemical Properties of Secondary Zinc Electrodes” Journal of the Electrochemical Society, 151 (2004) pp. A389-A393 describes La addition to zinc electrodes by preparation of Zn—La alloy electrodes [Zn(1−x)Lax, x=0.2-1]. It was observed that an enriched lanthanum oxide layer [La2O3/La(OH)3] formed, which prevented dissolution of the zinc electrode oxidation products and suppressed dendritic growth upon cycling. These benefits are allegedly due to La easily forming an oxide layer that cannot be reduced. The reference states that La addition has no effect on the anodic behavior of Zn electrodes: “La addition would have little influence on the discharge behavior of the zinc electrode in practical cells.” The focus of the prior art teaches towards optimization of a dendrite-suppressing oxide layer irrespective of the metal fuel, or in other words, an oxide of a differing metal than that of the fuel electrode.
Other inventions have focused on increasing the conductivity of a semi-conductive or insulating “semi”-metal fuels. Doped silicon as an active material in electrochemical cells has been proposed for example in U.S. Pat. No. 6,042,969 filed on Jul. 18, 1997 and in U.S. Patent Application Publication No. US 2011/0318657 A1 filed on Feb. 11, 2010. This prior art teaches to doping of a poorly conductive metal fuel itself, that metal being silicon.